Printing positives from a plurality of color photographic negatives

ABSTRACT

A METHOD OF PRINTING POSITIVES FROM A PLURALITY OF COLOUR NEGATIVES WHICH COMPRISES EXPOSING VIA THE SAID NEGATIVES PRINT MATERIAL TO PRINTING LIGHT CONTAINING AT LEAST TWO COLOUR COMPONENTS THE INTENSITIES OF WHICH, OR THE TIMES OF EXPOSURE OF THE PRINT MATERIAL TO WHICH, ARE CONTROLLED TO PROVIDE COLOUR CORRECTION WITH SUCH THAT PRINTING IS EFFECTED EITHER WITH FIXED COLOUR CORRECTION OR WITH COLOUR CORRECTION IN INVERSE PROPORTION TO THE RATIO OF INTEGRATED TRANSMITTANCES TO LIGHT OF SAID COLOURS OF THE NEGATIVE, THE PRINTING BEING EFFECTED WITH FUSED COLOUR CORRECTION WHEN THE SAID RATIO DEPARTS BY MORE THAN A PREDETERMINED PROPORTION FROM A PREDETERMINED VALUE, AND IN SAID INVERSE   PROPORTION VALUE WHEN THE SAID RATIO DOES NOT SO DEPART AND THE SELECTION OF THE TYPE OF COLOUR CORRECTION BEING EFFECTED BY INHIBITION OF SAID COLOUR CORRECTION IN INVERSE PROPORTION WHEN SAID RATIO OF INTEGRATED TRANSMITTANCE DEPARTS BY MORE THAN THE PREDETERMINED PROPORTION FROM THE PREDETERMINED VALUE.

D.- M. :NEALE PRINTING POSITIVES FROM A PLURALITY 5 June 15, 1971 OF COLOR PHOTOGRAPHIC NEGATIVES l0 Sheets-Sheet 1 Filed July 19, 1967 (X NOW/2m!) FIG. I

, Dmy/ (X72112; ll

June 15, 1971 o. M. NEALE PRINTING POSITIVES FROM A PLURALITY OF COLOR PHOTOGRAPHIC NEGATIVES 10 Sheets-Sheet I Filed July 19, 1967 fMy/a/d imam/11m f FIG. 3

D. M. NEALE PRINTING POSIT IVES FROM A PLURALITY June 15, 1971" OF COLOR PHOTOGRAPHIC NEGATIVES 10 Sheets-Sheet 9 Filed July 19. 1967 June 15, 1971 o. M. NEALE 3,535,029

PRINTING POSITIVES FROM A PLURALITY 0F COLOR PHOTOGRAPHIC NEGATIVES Filed July 19, 1967 10 Sheets-Sheet 10 5 70 fofm/Z 42 439 0/46/ FIG. l4

iM/m

United States Patent Ofiice 3 585,029 PRINTING POSITIVES FROM A PLURALITY OF COLOR PHOTOGRAPHIC NEGATIVES Denis Manktelow Neale, Ilford, Essex, England, assignor US. Cl. 96-23 21 Claims ABSTRACT OF THE DISCLOSURE A method of printing positives from a plurality of colour negatives which comprises exposing via the said negatives print material to printing light containing at least two colour components the intensities of which, or the times of exposure of the print material to which, are controlled to provide colour correction such that printing is eifected either with fixed colour correction or with colour correction in inverse proportion to the ratio of integrated transmittances to light of said colours of the negative, the printing being eifected with fixed colour correction when the said ratio departs by more than a predetermined proportion from a predetermined value, and in said inverse proportion value when the said ratio does not so depart and the selection of the type of colour correction being effected by inhibition of said colour correction in inverse proportion when said ratio of integrated transmittance departs by more than the predetermined proportion from the predetermined value.

This invention relates to colour photography and more particularly to the production of photographic positive colour prints from colour negatives.

In the following description of the invention reference is made to the accompanying drawings the natures of the figures of which are explained as each is referred to.

Consider a strip of colour negative material exposed to a succession of images of a typical subject (i.e. one Which integrates to grey), each individual exposure providing" double the integral of light against time relative to the preceding exposure. Such a strip is commonly referred to as an exposure-range of negatives. Relative to the optimum exposure for the particular negative material, the exposures form a geometric progression 36%, x /2, x1, x2, x4, x8. After the exposed strip has been processed to colour, the individual negatives, A, B, C, D, E, F, will have integrated transmittances to red light decreasing progressively. That is to say if negative A received one-quarter of nominal exposure, negative B onehalf, negative C received nominal exposure, negative D double, E four times and F eight times nominal, then negative A will have the highest integrated transmittance to red light and negative F the lowest. This is indicated graphically by curve I in FIG. 1. Curves II and III in FIG. 1 show that a similar progression is found in the integrated transmittances to green light and to blue light respectively.

Now suppose the strip of negatives A-F be printed with adjustment of the integrals against time of red, green and blue light intensity reaching the print material (elfected in any known manner) so that prints are provided which, after processing to colour, match each other as closely as possible in respect of colour and density.

Because almost identical prints are to be produced, the light intensity or exposure time used in printing a given negative has to be adjusted for each colour in approximately inverse proportion to the integrated transmittance of the negative to that colour.

FIG. 2 relates to the use of constant printing lamp in- 3,585,029 Patented June 15, 1971 tensity and variable time of exposure. In FIG. 2 curves, IV, V and VI represent measurements of red, green and blue light exposure times respectively required to produce closely matched prints from the exposure range of negative referred to in connection with FIG. 1.

The results shown in FIGS. 1 and 2 may be presented in the more general form shown in FIG. 3. In FIG. 3, the measured optimum printing time has been multiplied by the integrated transmittance, T, of the negative to light of the corresponding colour. The product represents the exposure integral, fI -dt, required for an optimum print, where 1,, is the light flux reaching the print material and t is time measured from start of exposure. It will be seen that, in the example chosen, the integral of green light (represented 'by curve VIII) remains substantially independent of negative transmittance. The integrals of red light and of blue light (curves VII and IX) show some dependence on negative transmittance. Some of the possible reasons for this dependence have been described by R. W. G. Hunt (1. Photographic Science, 8, pp. 212-219, 1960). It is nevertheless true that the integral remains more nearly constant than does either the corresponding negative transmittance or optimum printing exposure time. It is for this reason that automatic colour negative printers are designed to provide a pre-determined integral of light against time reaching the print material. Slope controls as described by Hunt (100. cit.) are used to make the pre-deterrnined value dependent on negative transmit- .tance to a relatively small extent. It will be noticed that in FIG. 3 the optimum integral even of blue light changes by less than 1.9 to 1 over a range of negative transmittance amounting to 6.4 to 1.

It is therefore to be understood that curves VII, VIII and IX are to be regarded as representing optimum integrals against time of light reaching the print material which integrals are substantially independent of integrated negative transmittance.

The foregoing may be regarded as a re-statement of the principles of printing colour negatives according to the known practice of integration to grey described in British patent specification No. 660,099. It is well known that this known practice has the desirable feature of providing compensation for those factors which aifect the ratios of integrated transmittances to red, green and blue light of negatives representing subjects which subjects themselves integrate to grey. For example a negative exposed to artificial light from a tungsten filament lamp will show a red-light transmittance which is lower, relative to its green-light transmittance, than that shown by a negative exposed to day-light. This subject has been discussed at length by Hunt (J. Photographic Science, 11 pp. 109-120, 1963).

When colour negatives are printed according to the integration-to-grey principle, difficulty is encountered with those negatives representing subjects containing an abnormally high proportion of one saturated colour. This type of subject is commonly referred to as a colour subject failure. Suppose for example, the subject is a girl wearing a bright red dress. Areas of the negative representing the dress will be almost unexposed to blue or green light. The blue and green-light transmittances of the negative will thus correspond to an otherwise identical negative of a girl wearing a black dress. The red-light transmittance will correspond to a negative of a girl wearing a white dress. In consequence, the red-light transmittance may be only half the transmittance normally corresponding to the accompanying blue and green-light transmittances. When such a negative is printed according to the integrate-to-grey principle, the time of exposure to red light is increased to maintain the predetermined integral against time of light reaching the print material. In consequence the print material receives an exposure to red light which is double the optimum value. It is clear that when a negative is to be printed corresponding to a colour subject failure, the exposure of the print material should ideally be eifected with the proportion of red, green and blue exposure times (or light intensities) unchanged from the proportion used to print a negative representing a subject which integrates to grey.

To print colour negatives to provide a high yield of acceptable prints, therefore, the negatives should be divided into two classes:

(a) Negatives representing subjects which integrate to grey. (b) Negatives representing colour subject failures.

The first class of negatives, class (a), should be printed according to the integration-to-grey principle. In practice it would also be acceptable to print these negatives by a method providing a lowered degree of correction, e.g. the

vmethod either of British Pat. No. 928,658 or of British Pat. No. 956,462.

The second class of negatives, class (b), should be printed with the proportion of red, green and blue light intensities incident on the negative and the proportions of exposure times of the print material to red, green and blue light approximating those proportions used in printing negatives in class (a).

This means that in printing a negative in class (b), the print material receives an exposure integral to light of at least one colour which integral is directly proportional to the integrated transmittance of the negative to light of that colour.

According to the present invention there is provided a method for printing positives from a plurality of colour negatives which comprises exposing via the said negatives print material to printing light containing at least two colour components the intensities of which, or the times of exposure of the print material to which, are controlled to provide colour correction such that printing is effected either with fixed colour correction or with colour correction in inverse proportion to the ratio of integrated transmittances to light of said colours of the negative, the printing being effected by fixed colour correction when the said ratio departs by more than a predetermined proportion from a predetermined value, and in said inverse proportion value when the said ratio does not so depart and the selection of the type of colour correction being effected by inhibition of said colour correction in inverse proportion when said ratio of integrated transmittances departs by more than the predetermined proportion from the predetermined value.

Preferably said inverse proportion value provides substantially constant integrals against time of light reaching the print material or said inverse proportion value is such as to compensate only partially for the effect of departures of said ratio from said predetermined value upon the integrals against time of light reaching the print material.

By fixed colour correction is meant that the ratios of the integrals against time of the product of the light intensity incident on the negative and light attenuation between negative and print material are in constant proportion.

Preferably also the said fixed colour correction is of one value when said ratio exceeds said predetermined value by more than said predetermined proportion and of another value when said ratio is less than said predetermined value by more than a predetermined proportion.

Colour correction may be effected by control of time of exposure of print material to each colour component of the printing light, or by control of the intensity of each such component. Usually the printing light will comprise three colour components and these are preferably such that said inverse proportion value provides integrals against time of light reaching the print material which would, if the light were distributed uniformly over the exposed area, lead to print material which after processing would be grey.

In a particular method using three colour components colour correction is applied between a first colour and a second colour on the principle that said fixed colour correction is of one value when said ratio exceeds said predetermined value by more than said predetermined proportion and of another value when said ratio is less than said predetermined value by more than a predetermined proportion, and is effected between the third colour and one of the said first and second colours according to the principle that fixed colour correction is provided when the ratio of integrated transmittances to light of said third and said one colour falls below a predetermined ratio by a proportion which is dependent on the ratio of transmittances of light of said first and second colours.

In one variant of the foregoing process, the exposure time to light of said third colour is adjusted in inverse proportion to the integrated transmittance of the negative to that colour. so far as is permitted by the limitation that the said exposure time shall not exceed the greater of two limiting values, which values correspond to a first predetermined proportion of the exposure time to said first and a second predetermined proportion of the exposure time to said second colour.

In another variant of the said process the intensity of said third colour component of printing light is arranged not to exceed the greater of two limiting values, which values correspond to a first predetermined proportion of the intensity of said first colour component of printing light and a second predetermined proportion of the intensity of said second colour component of printing light.

According to one embodiment of the present invention there is provided a method of printing colour positives from colour negatives which comprises exposing the print material to printing light comprising components of a plurality of colours and controlling the time duration of at least two of the exposures to said components so that they tend to approach integrals of light against time which correspond to predetermined values but are restricted by the limitation that the said times do not depart from a predetermined means ratio of exposure times by more than a predetermined proportion. The predetermined values referred to would normally be those which would afford a theoretical integration to grey.

In the method in which'colour correction is effected by control of time of exposure of print material to each colour component of printing light, one important variant is that'in which the exposure to light of each colour component is terminated at a time in inverse proportion to the integrated transmittance of the negative to that colour so far as is permitted by the limitation that the said exposure time is not permitted to exceed by more than a predetermined proportion a time which is in predetermined ratio to the time of exposure to another colour component. Within this variant the following conditions may exist.

(a) The exposure time to at least one colour component is related to a measured time interval by a first factor exceeding unit.

(b) Said measured time interval is related to a reference time interval by a second factor close to unity.

(c) Means are provided for adjusting said first and second factors in reciprocal relation, thereby varying the ratio of measured time interval to reference time interval without changing the ratio of exposure time to reference time interval.

(d) Said measured time interval is the time required for the voltage across a capacitor to reach a predetermined value when said capacitor is charged by current passed by a photoelectric cell receiving a sample of said one colour component of light of quality used for exposing print material.

(e) Means are provided for adjustment of said second factor, said means being responsive to the ratio of times of exposures to individual colour components of printing light.

(f) Said second factor is increased if said measured time interval corresponding to said one colour is less than the measured time interval corresponding to another colour and is decreased if more.

Adjustment may be made to the ratio of intensities of colour components of printing light so that negatives representing subjects integrating to grey cause prints to be made with substantially equal times of exposure to said colour components. The adjustment may be made by means responsive to the sequence of termination of exposures to individual colour components of printing light. Such means may include an electric motor connected to vary a resistance in series connection with a lamp providing one colour component of printing light, said motor being caused to reduce the value of said resistance when exposure to said one colour component terminates after exposure to another colour, said motor being also caused to increase the value of resistance when exposure to said one colour component terminates before exposure to said other colour.

Further it may be observed that where the method of colour correction by control of intensity of each colour component of printing light is employed a preferred variant is that in which the intensity of each colour component of printing light is adjusted in inverse proportion to the integrated transmittance of the negative to light of that colour, thereby maintaining at a constant value the intensity of each colour component of light reaching the print material, so far as permitted by the limitation that the ratio of intensities of two colour components of printing light shall not depart by more than a predetermined proportion from a predetermined value.

Although not restricted to such an embodiment, a particularly convenient embodiment is one in which printing exposures to red, green and blue light are substantially of equal duration when a negative is printed which represents a subject integrating to grey.

It has been suggested in B.P. 956,462 that wherein a colour printing system the printing times are unequal the exposure or exposures of shortened duration should be given at full colour correction while the exposure or exposures of longer duratin shuld be given at less than full colour correction. In this known process the factors mentioned vary inversely and continuously in that the extent of departure from full colour correction will be wholly dependant on the extent to which the exposures are of longer duration. That is to say, if a negative has an abnormally high density to red light, the exposure time to red light will be extended but the abnormality of red density will be undercorrected. Moreover, if the red density be further increased, the exposure time to red light will be further extended but the abnormality of red density will be under-corrected to a further extent. The method of the present invention differs essentially from this prior method in that the restriction on exposure times that do not depart by more than a predetermined proportion from a predetermined mean ratio interferes with and indeed over-rides any otherwise continuous relationship between exposure time and the extent of colour correction.

Thus an abnormality of red density is fully compensated by an extended exposure time to red light provided the extension does not exceed a predetermined proportion of the time corresponding to a normal red density. When the abnormality of red density produces an extension of exposure time corresponding to this proportion, however, any further increase in red density produces no further increase in exposure time to red light. Thus at this degree of abnormality of red density, colour correction ceases entirely. This may be seen more clearly from the following discussion in which convenient numerical data are assumed to illustrate typical examples of the use and advantages of the present invention.

To produce a pleasing print from a color negative, it

may be necessary to adjust the exposure to, say, red and to green light by increments not exceeding 10 percent (i.e. by 0.05 log exposure increment). This is particularly true of negatives depicting desaturated colours such as flesh tones and stone. R. W. G. Hunt has reported (Printing Colour Negatives, J. Phot., Sci., 11, pp. 109-120, 1963) that considerable'differences may arise in the relative densities to red and to green light due to the manufacturing tolerances and different histories of exposure and processing experienced by diif'erent rolls of film. Hunts FIG. 2 shows that an adjustment of about 0.35 log exposure unit is required to compensate fully the variations which may occur in only percent of rolls. (A still bigger range would be necessary to compensate for the remainder). If a printer is set up so that the middle of this range of adjustment has some special significance (e.g. the equal exposure-time condition proposed in Birtish Pat. No. 956,462), the range of colour control required for integration-to-grey on normal subjects is 10.175 log exposure unit in the red-to-green balance. If now, a correction factor of 0.7 is used as proposed in British Pat. No. 956,462, a negative requiring a correction of 0.175 log exposure unit for integration to grey will receive a correction of only 0.122 log exposure unit. The print will therefore remain under-corrected by 0.0525 log exposure unit and, as stated above, this is significant.

In the method of the present invention, any negative of a normal subject will be fully corrected (integrated-togrey) if the exposure times do not depart from the predetermined ratio by more than the predetermined proportion. If this proportion is :35 percent, say, any negative requiring a correction of less than $0.130 log exposure unit is fully corrected. However, a negative requiring a correction of 0.175 log exposure unit is then restricted to a correction of 0.130 log exposure unit. The degree of under-correction is then 0.045 log exposure unit, which figure represents small improvement on the method of British Pat. No. 956,462. It must be remembered, however, that the majority of negative have colour balances requiring corrections smaller than $0.130. log exposure unit. These will receive full correction according to the present method, but would show degrees of under-correction up to 1-0.039 log exposure unit using the method of British Patent No. 956,462.

The advantages of the present method become more clearly evident when negatives are considered which represent the so-called colour subject failures. Such a negative represents a subject containing an abnormally high proportion of one colour. For example, in a negative representing a girl in a white dress standing in front of a red motor car, the area of negative representing the red car will have a low density to blue and green light, but a high density to red light. Conventional photoelectric assessment of the relative densities of the negative to red, green and blue light may therefore indicate a red density higher by, say, 0.3 unit than the value normally corresponding to the observed green and blue densities. If such a negative is printed according to the integrate-togrey criterion, the exposure balance given to the print will be in error by 0.3 log exposure unit. If the loweredcorrection technique is used, them a correction factor of 0.7 will cause the print exposure balance to differ by :0.21 log exposure unit from the exposure balance given to a picture of a normal subject on the same roll of film.

In the present invention, the correction applied to the subject failure negative is confined within the same limits as the correction permitted to a normal negative. If these limits are :0.130 log exposure unit, it follows that a green and a red colour subject failure cannot be printed with corrections differing by more than 0.26 0 log exposure unit. In the integrate-to-grey method, the examples chosen would receive corrections differing by 0.60 log exposure unit. In the lowered-correction method, a correction factor of 0.7 would lead to exposure corrections differing by 0.42 log exposure unit. These results are summarised in Table 1 (which is set forth later herein).

A comparable table may be drawn up for corrections in the blue/yellow sense although the limits imposed on the blue/ yellow correction ma be wider than those used for the red/green correction, following the example of British Pat. No. 928,658 referred to above.

It was noted earlier that the colour balance of printing exposure may be critical to changes as small as 0.05 log exposure unit. This is particularly true of negatives representing subjects containing high proportions of desaturated colours, i.e. grey and near-greys. This type of subject normally integrates to grey and for this type of subject printing according to the integrate-to-grey criterion therefore leads to acceptable results. The same results will be produced according to the method of the present invention provided the negative is not of a balance requiring correction beyond the limits imposed. According to the known method involving a lowered correction factor, less than full correction is applied to negatives of abnormal colour balance. It follows therefore that by said known method less than the best degree of correction is given to the type of subject which is most critical in colour balance, that is, subjects whose negatives do not depart from optimum colour balance so as to require a correction of more than say $0.130 log exposure unit.

A colour subject failure usually represents a scene, a substantial proportion of which is of a saturated colour. When such a negative is printed in correct colour balance, the saturated colour will be represented by maximum density of at least one of the dyes of the print material and by minimum density of at least one dye also. For example, a saturated red is produced by maximum densities of yellow and magenta dyes and minimum density of cyan dye. The amount of cyan dye in the print is usually controlled by exposure to red light. An increase in ex posure to red light produces more cyan dye and this will tend to degrade the saturated red of the print, i.e. it will have the effect of adding grey to the colour, but it will not significantly affect the hue. Moreover, if the increase in exposure to red light is sufliciently small, the added amount of cyan dye will be small also because the gamma contrast of the print material is relatively low at low densities. Perhaps for this reason, it is found that the colour subject failure is much less sensitive to departures from correct balance than is the normal subject which integrates to grey. Even with the colour subject failure the best exposure balance is usually the same as that corresponding to a normal subject. From Table 1, it will be seen that the present invention provides a method which generally approaches this condition more closely than previously revealed methods.

It will thus be appreciated that it is necessary in carrying out the process of the present invention to establish certain pre-determined values. These values may vary in the printing of a sequence of negatives but are predetermined for each given negative in turn. Thus it is first necessary to predetermine desired integrals of light against time for the exposures to the three colours blue, green and red and these predetermined values will normally be those values which would produce a theoretical integration to grey. Secondly it is necessary to predetermine a desired mean ratio of exposure times to the three colours blue, green and red. The control system must then be set up to provide that while the exposure time for each colour continues towards the point at which the predetermined integral will be reached, it is checked at the point where further exposure would make the ratio of exposure times of the three colours differ from the predetermined mean ratio of exposure times by more than a pre-set extent.

It will be understood that the invention not only includes all the variations in the basic method of the invention as set forth above but includes the combinations of apparatus necessarily employed in carrying out the stated methods. Some such apparatus is referred to herein by Way of example. All such apparatus may be readily constructed, having a knowledge of the desiderata from readily available electronic components.

Having now stated the basis of the method, some indication will be given of the magnitude of the variables concerned.

When negatives of class (a) (subjects which integrate to grey) are to be printed in a printer giving time-wise exposure control to red, green and blue light components, the ratio of red to green exposure times should be varied to compensate for the following factors:

(i) Change in red/ green time ratios associated with a set of optimum prints from an exposure range. In the example shown in FIG. 2 this ratio changes by less than 115% of a mean value.

(ii) Change in red/green time ratios required to compensate for differences in negatives exposed to light sources between colour temperatures 3,200 K. and 5,000" K. The red/green light emissions of these sources vary in ratio by about 10%. A negative material of gamma 0.7 will thus show red-green density differences varying, due to this cause, by about 0.7 log 1.4 density units, i.e. 0.1 density unit. Compensation requires changes in red/ green printing time ratio of about 25% or i12 /2% from a mean value.

(iii) Change in red/ green time ratios required to compensate for manufacturing and processing differences between ditferent pieces of negative material. This change may correspond to about i15%.

From the foregoing, it will be seen that the red/green printing time ratio for a class (a) negative should never depart from a particular mean ratio by a factor exceeding the product of the individual factors due to (i), (ii) and (iii) above. The maximum departure from the mean ratio will thus be by a factor 1.15 l.125 1.l51.55 to 1. In practice the great majority of class (a) negatives will incur a factor much closer to unity.

If negatives of class (b) (colour subject failures) are printed according to the integrate-to-grey principle, the changes in red/green time ratio correspond to factors larger than the maximum factor of 1.55 to 1 just mentioned. Factors of 2.0 to 1 are quite common. In practice, the distinction between negatives of class (a) and class (b) may be made simply by observing whether a predetermined ratio of red and green exposure integrals requires a departure from a typical ratio of red/ green exposure times by a factor exceeding a particular value. The value implied by the foregoing is 1.55. Because class (a) negatives soldom produce changes of this extent, it may in practice be advantageous to use a smaller value, e.g. 1.35.

It is well known that negatives on a single strip, exposed under similar conditions and processed together require the same colour correction to produce acceptable prints, irrespective of Whether or not they represent colour subject failures. Known types of automatic colour print ers produce unsatisfactory results because the colour correction applied to a colour subject failure is different from that given for a subject integrating to grey. In the present invention, therefore, a negative in class (b) (i.e. a negative representing a colour subject failure) is detected by virtue of anabnormal proportion of integrated transmittances to light of any two colours, red, green and blue. The colour correction applied in respect of these colours is then confined to a value known to be acceptable for negatives representing subjects integrating to grey.

This principle is further clarified by FIG. 4 in which the abscissa represents on a logarithmic scale, the ratio of integrated transmittances, T and T to light of two colours (red and green) multiplied by a constant k This constant k represents the average value of 7 /7 for a large number of negatives. The ordinate in FIG. 1 represents the color correction between red and green light. This is plotted on a logarithmic scale as the ratio of the individual products of light intensities I and I 9 incident on the negative and exposure time t and r of the print material.

The factor k represents the average value of for the same large number of negatives.

In FIG. 1, the broken line represents known practice according to B.P. 660,099, i.e. printing according to the integrate-to-grey criterion. In this an abnormality of integrated transmittances in the ratio 2 to 1 produces a change in I t /I t in the ratio 2 to l.

The dotted line in FIG. 4 represents known practice according to RP. 956,462 or B.P. 928,658 i.e. printing with a degree of correction corresponding to 0.7 of the value corresponding to the method of B.P. 660,099.

The solid line WXY Z in FIG. 4 exemplies the method according tothe present invention. The central section XY indicates that negatives in class (a) are to be printed according to B.P. 660,099. The sections WX and ZY indicate that negatives in class (b) will be printed with a fixed degree of colour correction the direction of which corresponds to the direction indicated by the known methods quoted.

Alternative arrangements are clearly possible. For example class (a) negatives might be printed with a lowered degree of correction. Such a system would be represented by the line WX'Y'Z in FIG. 4. Furthermore, although WX and YZ have been drawn parallel to the abscissa, they could each have a small gradient, either positive or negative. Such alternative arrangements are in practice more complicated to execute and less satisfactory in performance.

Although the foregoing principles provide a simple means of controlling the ratio of red and green exposures, care must be exercised in extending the principle to include control of exposure to blue light. FIG. 2 shows that the ratio of optimum printing exposure times to blue and green light changes in an exposure range of negatives more than the ratio of red to green exposure times. The ratio of blue and red exposure times changes still more widely. The ratio of blue exposure time to'either other exposure time must therefore allow for the following factors:

(i) Change in blue/red time ratios in an exposure range. In FIG. 2 this ratio changes by 240% about a mean value.

-(ii) Change in blue/red time ratios required to compensate for differences in negatives exposed to different light sources. Compensation may require a change in blue/red printing time ratio of 225%.

(iii) Changes in blue/red time necessary to compensate for manufacturing and processing variations may amount to 115%.

10 this is permitted the resulting prints are often of too low a density.

An embodiment overcoming these difficulties is as follows:

The red and green light exposure times are controlled as already described so that their ratio shall not depart from a predetermined value by more than a predetermined factor (say 1.35 to 1). Variations in blue transmittance (or exposure time) are not allowed to affect the individual red and green exposure times or their ratio. The blue light exposure time is limited not to exceed by more than a factor xa time which the greater of two times viz y times the red exposure time or 2 times the green exposure time. The values of y and z correspond to the average values of blue/red and blue/ green printing exposure times for a large number of negatives. The factor x may be in the range 1.0 to 1.5. Work relating to FIG. 2 showed good results with a value of x=1.20.

FIG. 5 represents the dependence of blue/ green colour correction provided by such a printer upon the ratio of integrated green and blue transmittances of the negative. FIG. 5 should be compared with FIG. 4. In FIG. 5 it will be noted that colour correction is in inverse proportion to green/blue transmittance for values of where k is the average value of T /T for a large number of negatives. Colour correction ceases to be dependent on T /T at a value of T /T which never exceeds (l.2 1.35)k =1.62/k The value of T /T at which the limit is imposed depends on the value of k T /T since this determines the relation between red and green printing exposures in accordance with the line WXYZ in FIG. 4.

If a printer is adjusted so that for average negatives the red, green and blue exposure times are all equal, then y=1, and z=1. In a printer in which red, green and blue light exposures proceed concurrently, this leads to a particularly simple embodiment. If x=1.2, exposure to blue light is then terminated (if the blue light exposure integrator has not already cause it to terminate) at a 20% extension of the time of termination of both red and green light exposures.

The performance of such a printer will now be discussed to show how it meets practical requirements.

(i) The printer will be adjusted to provide a dead-heat condition on negatives in the middle of the exposure range, i.e. on the negative which received twice nominal exposure.

The printer performance on an exposure range of negatives may be summarized by the following table (Table II) deduced by reference to FIG. 2.

TABLE II.-EXIOSURE RANGE OF NEGATIVES EXPOSED TO LIGHT OF NO RMAL COLOUR TEMPERATURES Proportion to which colour subject failure must increase printing time to incur Printing exposure times (relative Limitations on print- Hence negatives in class (a) might require correction of blue/red printing time ratio corresponding to the product of these individual factors, i.e. by a combined factor of 1.40 l.25 1.l5=2.0 to 1.

It is not satisfactory to use as a means of discrimination between negatives in class (a) and class (b) an abnormality of transmittance ratio of 2 to 1. Colour subject failures of this extent or less would be placed in class (a) and the resulting prints would be unsatisfactory.

Moreover it has been found in many cases undesirable to allow an abnormally high blue transmittance to lead to a shortening of red and green light exposure. When The bracketed value of 1.442 indicates that the x8 negative would produce an optimum print with a blue printing exposure 1.44 times the duration of the red exposure time. But the blue exposure is terminated at 1.2 times the green exposure which is itself 1.17 times the red exposure. Hence the actual blue exposure is 1.17 1.20t i.e. 1.10 times the red exposure time. Thus on the x8 negative, the blue exposure time will be only 97% of optimum. The deficiency of 3% is in practice quite unobservable, and the protection against blue colour-subject failures associated with dense negatives is highly desirable.

11 (ii) Similarly, Tables III and IV may be constructed to sumarize performance when printing negatives exposed to light of high and low colour temperatures.

TABLE IIL-EXPOSURE HIGH COLOUR TEMPERATURE 12 FIG. 14 is a schematic block diagram showing details of the potentiometers and time ratio controllers.

FIG. 6 represents an automatic colour negative printer RANGE OF NEGATIVES EXPOSED TO LIGHT OF Proportion to which colour subject failure Printing exposure must increase printing A x8 negative exposed to light of a high colour temperature will receive a blue-light exposure 12% less than optimum. This is just observable, but not serious. In fact 12% represents only about one-half of the step pro- -vided between consecutive blue colour-correction buttons on an automatic printer.

according to the known art. In FIG. 6 light from three lamps 1, 2, 3, passes through a diifusing screen 4 to enter an integrating box 5, at the lower end. The upper end of the integrating box is provided with a diffusing screen 6, so that a negative 7, shall be uniformly illuminated and an image thereof be formed by lens 8, onto photo- TABLE IV.EXPOSUIIKJ%VI;ANGE OF NEGATIVES EXPOSED TO LIGHT OF COLOUR TEMPERATURE Proportion to which colour subject failure Printing exposure time s(relative Limitations on printmust increase printing time to incur Negative to shortest) ing exposure times limitation exposure (x nominal) R G B R G B R G B 0.25 2.1%, 1.64% t 1.35t 1.35tr 1.2t 1.01 1.80 2.62 1.34m, 1.14m, t 1.?5t 1.35t, 1.2tr 1. l5 1. 59 1. 61 1.01t t, 1.0812,; 1.35% 1.3m. 1.2tr 1. 34 1. 36 1. 21

From Table IV it is seen that optimum exposure times are provided for all negatives in class (a) which have been exposed to light of low colour temperature. For these negatives, protection against red colour subject failures is good. Protection against green or blue colour subject failures is least satisfactory on the under-exposed negatives.

(iii) From Table II-IV, it will be seen that in most circumstances an additional exposure time varia tion will be permitted to compensate for abnormalities in transmittance ratios due to manufacturing or processing variations. Cases have a 15% increase in exposure time would not be provided are as follows:

(a) Red exposure of x 0.25 negative exposed to low colour temperature source.

(b) Blue exposure of x 8.0 negative exposed to normal colour temperature source.

(c) Blue and green exposures of x 8.0 negative exposed to high colour temperature source.

These combinations of variables occur so rarely as to be of little consequence.

The following drawings will serve to illustrate the invention.

In the drawings FIG. 6 is an automatic colour negative printer according to the known art shown diagrammatically.

FIG. 7 is a schematic block diagram of one of three similar channels of control circuitry according to the same known art.

FIG. 8 is a schematic block diagram of the exposure control circuit of a known automatic colour negative printer.

FIG. 9 is a block schematic diagram of the additional circuit to be used with that of FIG. 8.

FIG. 10 is a diagram of the circuit arrangement used to provide a time ratio controller.

FIG. 11 is a block schematic diagram of a circuit for making an automatic adjustment of the relative intensities of red, green and blue-light components.

FIG. 12. is a circuit diagram of the arrangement whereby motors may be made to adjust the printer by rotation.

FIG. 13 is a schematic block diagram of one form of exposure control circuit showing the location of the time ratio controllers.

sensitive print material 9. Lamps 1, 2, 3, are provided with colour selective filters 11, 12, 13, each passing a different one of three bands of the spectrum, red, green and blue respectively. An inclined mirror 10 directs light from lamp 1 and passing the red filter 11 onto diffuser '4. An included mirror 14 directs light from lamp 3 passing the blue filter 13 onto diffuser 4. An inclined part-reflector 15 directs to lens 16 a proportion of light passing negative 7 and lens 8. Light reaching lens 16 enters a photocell box 17.

FIG. 7 represents in block schematic one of the three similar channels of control circuitry whereby according to known art a photocell 21 in the photocell box 17 of FIG. 6 controls a shutter in one of the planes 18, 19, 20 (FIG. 6) to terminate exposure of print material 9 to light of one of the colours red, green, blue. A similar channel of control circuitry controls exposure to light of each of the three colours, red, green, blue.

Referring to FIG. 7, in the passive condition, current from power supply 30 holds actuator 26 energised so that shutter 27 is closed and light from the printing lamp 1, 2, or 3 in FIG. 6) is unable to reach the print material 9. Actuator 26 may comprise a solenoid connected in series with a thermionic valve or a transistor. T0 expose a print, switch S1 is closed. Current from power supply 30 then sets the bistable trigger device 24 to an off (0) condition. The OR gate 25 then receives two 0 inputs and delivers a 0 output to actuator 26. In consequence, actuator 26 becomes de -energised and and the shutter 27 opens under control of a return spring (not shown). Opening of shutter 27 allows printing light to fall on the print material. A proportion of the printing light also falls on photocell 21 giving rise to a current proportional to the intensity of printing light reaching photocell 21. Current from photocell 21 passes to a current integrator 22, usually a capacitor. Integration of current by the integrator 22 produces a voltage increasing with time. The output voltage of integrator 22 is applied to the voltage adder 23. The other input to the adder 23 is a voltage obtained from a voltage reference source 28 and attenuated by an adjustable potentiometer 29. The two inputs to the adder are of opposite polarity. Accordingly, the output of the adder falls to zero when the two inputs are equal in magnitude.

The Output of the adder 23 is applied to trigger device 24. So long as the output of integrator 22 is smaller than the output of potentiometer 29, adder 23 delivers an output causing trigger circuit 24 to apply a zero signal to OR gate 25 and in consequence actuator 26 remains in a de-energised condition. When photocell 21 causes integrator 22 to charge sufficiently, the output of adder 23 falls to zero, trigger 24 delivers a (1) output to OR gate 25, actuator 26 becomes energised and shutter 27 closes to terminate exposure of print material to light of that colour to which photocell 21 responds.

It will be understood that in automatic colour printers according to the known art three circuits similar to FIG. 7 are used, each corresponding to a separate colour red, green or blue. For each such circuit a red, green or blue colour selective filter is placed in the path of flight reaching photocell 21.

In the known art it is also customary to provide a switch S which may be closed manually so that, it required, actuator 26 may be energised prematurely to terminate exposure of print material. Commonly only one power supply, 30, may be required in connection with all three channels and switches S and S may be common to all three exposure control channels.

FIG. 8 represents the exposure control circuit of such an automatic colour negative printer according to the known art. The numbered circuit elements in FIG. 8 correspond to elements of like number in FIG. 7 and the suffixes r, g, b, relate to the red-, greenand the blue-light exposure control channels.

Having now described a known form of automatic colour negative printer, an embodiment of the present invention will be described as applied to such a printer. FIG. 9 represents in block schematic the additional circuit to be used in connection with that of FIG. 8 in order to provide required limitations on the ratios of times of printing exposures to red, green and blue light. FIG. indicates the circuit arrangement used to provide a time ratio controller of the type indicated by blocks 37 and 39 in FIG. 9.

In FIG. 10 a bistable circuit 41 provides alternative fixed positive or negative output voltages in response to changes of input voltage applied to the input terminal 40. If the input voltage is zero (0), the output voltage applied to diodes, 42, 44 is positive. If an input voltage (1) is applied, the output voltage becomes negative. In the standby condition (i.e. before printing commences) the input at 40 is in the 1 condition. The output from bistable 41 is then negative and a current is drawn through diode 44, resistance 45 and diode 46 from earth. The current so drawn produces only a small voltage drop across diode 46 and the input to phase-inverting amplifier 47 may therefore be considered to be held substantially at earth potential. While the input to amplifier 47 is held at earth potential in this way, the output of 47 (and hence the input to trigger 49) is also substantially at earth potential, i.e. in a 0 condition. With a 0 input condition, the trigger circuit 49 provides a 1 output at terminal 50.

When a 0 input is applied to terminal 40, bistable 41 provides a positive output voltage to diodes 42 and 44. Diodes 44 and 46 then cease to conduct and a current flows through diode 42 and resistance 43 to raise the potential of the input of amplifier 47. Since amplifier 47 provides phase-inversion, any rise of input potential produces a fall in output potential and a feedback current flows through capacitor 48 to oppose (incompletely) the rise in input potential of amplifier 47 The configuration of phase-inverting amplifier 47 and feedback'capacitor, 48, will be recognised as the well-known Miller integrator circuit. It provides that, while bistable 41 provides a positive output voltage, the negative output voltage of amplifier 47 increases linearly with time. The rate of this increase, dV/dt, is given by h.V /R where h is a constant, V is the positive output voltage of bistable 41 and R is the value of resistance 43. So long as the output voltage of amplifier 47 remains negative, trigger 49 delivers zero (0) output to terminal 50.

When a 1 input is again applied to input terminal 40, the bistable delivers a negative output voltage, V to diodes 42 and 44. Diode 42 then ceases to conduct and a current is drawn through diode 44 and resistance 45 to reduce the potential of the input of amplifier 47. The feedback capacitor 48 again acts to oppose incompletely the change in input potential to amplifier 47. Accordingly the output potential of amplifier 47 rises (towards earth potential) at a rate dV/dt given by h.V /R where R is the value of resistance 45. When the potential of the output of amplifier 47 once more reaches earth potential, the input to trigger 49 is again 0 and trigger 49 again delivers a l at output terminal 50.

It will be understood from the foregoing that if the input voltage to terminal 40 changes from 1 to 0 the output voltage at terminal 50 changes to 0 immediately. If the input voltage to terminal 40 thereafter returns to 1 after a time t, the output voltage at terminal 50 will return to 1 after a time t(1+q/ p) where If V and V are made equal, q/p=R /R The time ratio controller shown in FIG. 10 may thus be used to define an interval t(l+q/p) which begins in response to an input voltage change, the duration of the interval being proportional to the duration t of the input voltage change and equal to the interval 1 extended by a prescribed proportion q/p of t.

FIG. 9 shows how two such time ratio controllers may be used in conjunction with a printer corresponding to FIG. 8 in order to impose certain restrictions on the ratios of exposures to red, green and blue light of the print material. In FIG. 9 terminals 31', 32, 33, 34, 35', are connected to terminals 31, 32, 33, 34, 35, respectively of the circuit in FIG. '8. When switch S1 (FIG. 8) is closed to initiate printing, therefore, the outputs OR gates 25r and 25g change from the l to the 0 state. Both inputs to OR gate 36 therefore change from 1 to 0 and hence the input to the time ratio controller also changes from 1 to 0. Suppose now that the redlight integrator 22r produces an output voltage equal and opposite to the output of potentiometer 29r and suppose this happens before the corresponding event occurs in respect of components 22g and 29g. Then the OR gate 25r Will produce a 1 output to operate actuator 26r and close shutter 27r. The 1 output from gate 251- Will be applied to OR gate 36 and so produce a 1 input to time ratio controller 37. The input of 37 has then remained in the 0 condition for a time t, equal to the duration of the red light exposure. The output of the time ratio controller 37 therefore returns to a 1 condition after a total time t,(1+q/p). If the greenlight exposure has not been completed by such time due to the action of integrator 22g, the generation of a 1 at the output of time ratio controller 37 causes a 1 to appear at an input to OR gate 25g and thereby to terminate the green-light exposure. The green-light exposure time is thus prevented from exceeding the red-light exposure time by more than the proportion (1+q/ p) :1.

Since the circuit connections are symmetrically disposed relative to the redand green-light exposure control channels, it will be understood that should the greenlight exposure integral be reached before the red, the time ratio controller will prevent the red-light exposure time from exceeding the green-light exposure time by more than the same proportion, (l+q/p):1.

If the intensities of red and green components of printing light are proportioned so that the average redand green-light exposure times are substantially equal, then it is found satisfactory to use a ratio q/p=0.2, that is (l-i-q/ p) :12. The combination of the circuit of FIG. 9

with that of FIG. 8 :will then ensure that the red exposure time cannot exceed the green by more than 20% nor can the green exposure time exceed the red by more than 20%. The ratio of redand green-light exposure time is thereby inhibited from changing sufliciently to compensate for departures in the ratio of integrated transmittances of the negative to red and green light exceeding i008 log unit from an average value.

It would clearly be possible to arrange in analogous manner for a similar symmetrical restriction on the ratio of red/blue or green/blue exposure times. In practice it is found preferable, however, to arrange that the blue exposure times be restricted to depend on red and/r green exposure time but that the blue exposure time shall impose no restriction on the red or green exposure times. The embodiment shown at FIG. 9 provides this preferred result. When S is closed to initiate printing, outputs of OR gates 25r and 25g change from 1 to 0. Consequently the output of AND gate 38 also changes from 1 to 0 until both red and green exposures are complete. Only when both 25r and 25g again deliver 1 outputs does AND gate 38 deliver a 1 to time ratio controller 39. After a proportioned time delay produced by the time ratio controller 39, an output 1 is produced and applied to OR gate 25b to terminate the blue-light exposure if it has not already been terminated by action of integrator 22b. In this way the blue exposure time is inhibited from exceeding more than a prescribed fractional extension (x-l) of the longer of the two exposure times, that to red light or that to green light. In practice this extension may be made as small as or even less.

It will be noted that if a negative be printed which has been exposed to light of abnormally low colour temperature, the blue-light exposure will be terminated by integrator 22b considerably in advance of termination of exposure to either red or green light. The arrangement shown in FIG. 9 intentionally avoids restricting the redor green-light exposure to a prescribed extension of the blue exposure time since such restriction would inhibit full compensation by the integrators 22r, 22g and 22b for the effects of exposing the negative to light of low colour temperature.

The embodiment shown so far described with reference to FIG. 9 may even so produce excessively red prints from negatives exposed to light of abnormally low colour temperature, e.g. light from a clear flashbulb or photo flood lamp. This arises because, whereas the exposures to blue and to green light may be completed by action of integrators 22b and 22g respectively, the exposure to red light is terminated at the prescribed time extension (e.g. of the green light exposure time. A preferred embodiment overcomes this shortcoming by arranging that a 1 signal appearing at terminal 34 be used to energise a relay 51, which closes contacts 52, to connect an additional resistance 53, in parallel With resistance 43 of the time ratio controller 37. When negatives of normal colour balance are printed, the blue exposure is terminated only very slightly in advance of red or green (if at all). In consequence, resistance 43 is shunted for very little (if any) of the time for which the output of bistable 41 is positive. The time ratio determined by 37 is then affected little or nothing by the shunting effect of resistance 53. When a negative is printed which has been exposed to light of a low colour temperature, however, the blue-light exposure is terminated by integrator 22b considerably before either the greenor red-light exposure is complete. Consequently resistance 53 shunts resistance 43 for a substantial proportion of the time for which the output of the bistable 41 (in unit 37) is positive. The value of resistance 53 is made lower than the value of resistance 43. Consequently while bistable 41 is positive, the input to amplifier 47 is raised to a higher voltage than would be the case without resistance 53 and the extension of the green exposure time determined by controller 37 (and limiting the duration of red exposure time) is substantially increased.

Specifically, if the value of resistance 53 is one-fifth that of resistance 43, then a blue exposure terminating at one-half the time of the green exposure will cause the permitted extension of red exposure beyond green to be increased from the normal 20% to 3.5 20%, i.e. 75%.

Although it is possible to operate the invention as so far described by adjusting manually the relative intensities of red, green and blue-light components in printing light, it is advantageous to employ a formof the invention in which such adjustment is made by automatic means. The automatic adjustment is designed to ensure that for typical negatives of a prescribed class substantially equal times are required for integrators 22r, 22g, 22b to operate the corresponding triggers, 24r, 24g, 24b. One method of making such automatic adjustment will be described with reference to FIG. 11.

The circuit arrangement shown in FIG. 11 may be used in conjunction with the known type of printer control circuit shown in FIG. 8. In FIG. 11, terminals 31", 32", 34", 54", are connected to terminals 31, 32, 34, 54, respectively 'in FIG. 8. (This may be in addition to the connection of the circuit of FIG. 9 to certain of these terminals.)

When S (FIG. 8) is closed to initiate printing, a 1 signal appears at terminal 54" and is differentiated by the combination of capacitor 64 and resistors 63, 73. The differentiated signal is in the form of an exponential pulse and is applied to bistable circuits 58 and 68 to cause them to deliver zero (0) outputs and so to de-energise relays 59 and 69. Associated with relay 59 are contacts 60 which control the direction of rotation of a split-phase induction motor 62. Similar contacts 70 are associated with relay 69 and a second split-phase induction motor 72. For descriptive purposes, it is assumed that the contacts 60, 70, have been drawn in a condition corresponding to the de-energised condition of relays 59, 69. Also it is assumed that, in this condition, closure of contacts 82 would cause the shafts of motors 62, 72, to rotate in a clockwise direction if a suitable alternating current supply be connected to terminals 83 and 83a. Operation of contacts 60 or 70 would then apply the alternating current supply to the opposite pole of phase-splitting capacitor 61 or 71. In consequence the associated motor 62 or 72 would rotate in the reverse (i.e. counter-clockwise) directions. It will be noted, however, that when S1 is closed to initiate printing, relay 81 is de-energised and its associated contacts 82, are open, as shown. Hence neither motor 62 or 72 rotates immediately.

Before contacts 82 close, the direction of rotation of each motor 62, 72, is pre-selected by relays 59, 69. The manner in which this is done is as follows: At the beginning of the printing cycle, terminals 31" and 32" receive 0 signals from the OR gates 26; and 26g (FIG. 8). When exposure to red light is terminated, the signal on 31" changes to a 1 condition. The 1 signal is differentiated by capacitor 55 and resistor 56 to provide an exponential pulse which is applied to AND gate 57. If the green exposure has already terminated, a 1 signal will be present at terminal 32" also and gate 57 will pass a pulse to bistable 58 switching the output to a 1 condition and so energising relay 59'. If, on the other hand, the exposure to green light terminates after the exposure to red light, then the gate 57 will not respond to the exponential pulse passed by capacitor 55. Consequently relay 59 will in that event remain de-energised. It will thus be seen that if contacts 82 are closed at some time after completion of both redand green-light exposures, motor 62 will rotate in a clockwise direction should the green exposure have been longer than the red and the motor will rotate counter- 17 clockwise should the green exposure have been shorter than the red.

Components 65-69 operate in a manner closely analogous to that of components 55-59. The AND gate 67 has an extra input, however, and hence relay 69 becomes energised if the exposure to blue light continues after termination of exposure to both the colours, red and green. Thus if the blue exposure time is the longest, motor 72 will rotate counter-clockwise on closure of contacts '82.

On closure of switch S1, the exponential pulse passing capacitor 64 is applied also to monostable circuit 74. On

receiving the pulse, monostable 74 immediately changes the state of its output from a 1 to a 0. After a fixed time interval t the output of 74 returns to a l condition. When each OR gate 251', 25g, 25b (FIG. 8) has produced a 1 signal to terminate printing of each colour, red, green, blue, the 1 signals appearing at terminals 31", 32", 34", cause AND gate 75 to produce a 1 at its output. The output from gate 75 is differentiated by capacitor 77 and resistors 76, 78, to produce an exponential pulse which is applied to AND gate 79. If this pulse is received while the output of monostable 74 is in the condition, gate 79 can produce no output and contacts 82 will not be closed to cause motors 62, 72, to rotate. If, on the other hand, the exponential pulse passing capacitor 77 occurs after the output of monostable 74 has returned to the 1 condition, then gate 79 passes a pulse to monostable 80. On receipt of this pulse, the monostable 80 changes from a 0 state to a 1 state at its output and relay 81 is thereby energised for a time interval of duration t defined by monostable 80.

It will thus be understood that, for each negative requiring a printing exposure exceeding the time t motors 62 and 72 are caused to rotate for a time r and in directions dependent on the sequence of termination of the red, green and blue light exposures. Moreover it will be noted that very short printing exposures (such as occur when the printer is operated with no negative, or with an unexposed negative, in the gate) lead to no rotation of motors 62, 72.

FIG. 12 shows one simple arrangement whereby rotation of motors 62 and 72 may be made to adjust the printer so that, for typical negatives requiring printing exposures in excess of time t the exposure times to red, green and blue light are substantially equal. In FIG. 12, an alternating current supply is connected to terminals 88 and 89 so that current may flow from terminal 88, through the sliding contact 86 of potentiometer 84, through one part of the potentiometer winding to terminal 91, through lamp 3 providing the blue component of the printing light and thence to terminal 89. Current flows also from sliding contact 86 through the other part of the winding of potentiometer 84 to terminal 90 and thence to sliding contact 87 of potentiometer 85. Current passes thence through the two parts of the winding of potentiometer 85, leaving by terminals 92 and 93 to pass through lamps 2 and 1 to supply terminal 89. Lamp 2 provides the green component of printing light and lamp 1 the red.

It will be understood that the relative intensities of the red, green and blue light components of printing light (obtained from lamps 1, 2 and 3) depend on the positions of potentiometer sliding contacts 86, 87. These are coupled to the shafts of motors 72, 62, as indicated by broken lines in FIG. 12. Thus, if blue is the last colour to terminate printing, the circuit of FIG. 11 causes motor 72 to rotate potentiometer contact 86 in a counter-clockwise direction for a time t This rotation reduces the resistance between 86 and 91 which is connected in series with lamp 3. At the same time, the rotation of 86 increases the resistance between 86 and 90 and connected in series with lamps 1 and 2. Consequently for the next printing exposure the intensity of blue light is increased and the intensities of red and green light are decreased. By choice of motor speed, the duration of interval r and the resistance of potentiometer 84 it is arranged that the intensity ratio of blue light to red light changes only in the order of 1% for a single operation of relay 81 (FIG. 11). A dozen or more typical negatives may therefore have to be printed before contact 86 reaches a position in which the intensity of blue printing light has been increased so far that the blue exposure times are reduced to substantial equality with the exposure times to red and green light. Thereafter contact 86 will move in opposite directions as successive negatives or groups of negatives are printed. The mean position of contact 86, will, however, be automatically adjusted to maintain substantial equality of printing exposure times between the three colours.

In the coupling between motor 72 and contact 86 it is useful to provide a degree of backlash equivalent to the movement of contact 86 required to effect a change of about 10% in the ratio of blue and red light intensities.

In a closely similar manner, the circuit of FIG. 11 causes motor 62 to move contact 87 of potentiometer so that, for the typical negative, the red and green printing times are made equal. If the green printing times are longer than the red, the intensity of lamp 2 is increased and that of lamp 1 decreased.

Foregoing embodiments relate to a known type of printer using separate lamps as sources of the red, green and blue light components of printing light. In such a printer it is convenient to arrange for substantially equal exposure times to the three colour components of printing light, red, green and blue. In an important alternative known type of printer, however, the three components of printing light are derived from a single lamp. In this alternative type of printer it is not usually convenient to make provision for continuous adjustment of the relative intensities of red, green and blue components of printing light. Accordingly an alternative embodiment will be indicated to show that the invention is not confined in its applicability to a single known type of printer and, in particular, that it is not confined to a method of printing in which the times of exposures to red, green and blue light are maintained substantially equal.

The electronic exposure control circuits of the said alternative known type of automatic colour negative printer may in general be in accordance with the known art shown in FIG. 8. In order to apply the invention to such a printer, time ratio controllers 94, 95, 96, are added to the conventional circuit between the OR gates, 25r, 25g, 25b, and the actuators 26r, 26g, 26b. In other respects the circuit of FIG. 7 is to be considered unchanged and accordingly only the relevant parts of the modified form are indicated in FIG. 13. The circuit of FIG. 11 may be used in conjunction with that of FIG. 13, terminals 31, 32, 34, 54, in FIG. 13 being connected to correspondingly numbered terminals in FIG. 11 as previously indicated.

FIG. 11 also functions substantially as described previously, i.e. operation of relays 59, 69, is determined by the sequence in which OR gates 25r, 25g, 25b, change from the O to the 1 state. If time ratio controllers 9'4, 95, 96 are adjusted to provide other than identical values of q/ p, the sequence in which red-, green-, and blue-light exposure are terminated will in general be different from the sequence in which gates 25r, 25g, 25b, change from the 0 to the 41 state.

When used in connection with the circuit of FIG. 13, the circuit of FIG. 11 is used in relation to the circuit arrangement of FIG. 14 (rather than the circuit of FIG. 12). FIG. 14 shows details of potentiometers 29r, 29g, 29b, and parts of the time ratio controllers 94, 95, 96. FIG. 11 shows also how certain of these components are controlled by motors 62 and 72.

In FIG. 14 it is shown that the potentiometer 29g may comprise two fixed resistors 97, 98. Adjustment of exposure integral of print material to green light is produced by manual adjustment of the setting of sliding contact 99 on variable resistance 100, in the time ratio controller 95, of the green exposure control channel. Resistor 43g 19 in controller 95 corresponds to resistor 43 in FIG. 10. If in FIG. 14 the variable resistance 100 has a maximum value R then the proportion 11 brought into circuit by sliding contact 99 is tp R This resistance, R corresponds to resistance 45 in FIG. 10. By adjustment of \p the duration of green exposure may be adjusted to that value producing the required amount of magenta dye in a print after processing.

In the red exposure control channel, controller 94 contains a variable resistance 106 of maximum value R The proportion 0,. of R brought into circuit by sliding contact :105 may be varied manually to that value producing the required amount of cyan dye in a print after processing.

Similarly in controller 96 in the blue exposure control channel, the proportion 1, 1 selected by sliding contact 110 of variable resistance 111, maximum value R may be adjusted to produce the required amount of yellow dye in a print after processing.

when a succession of prints is made, motor 62 will move clockwise or counter-clockwise depending whether gate r changes from the 0 to the '1 condition before or after gate 25g. Motor 62 is mechanically coupled to move the sliding contact 101 over the winding 102 of potentiometer 29r. The reference voltage delivered to adder 23r is determined by the proportion 0 of the winding 102 selected by contact 101. If, in exposing a print, gate 25r changes from the *0 to the 1 condition after gate 25g, then relay 59 (FIG. 11) will be energised to cause motor 62 to rotate in a counter-clockwise direction and so to reduce the value of 0,. On a subsequent print, the smaller value of 0,. will cause trigger 24r to produce a 1 signal with a smaller voltage delivered from integrator 22r. Consequently the operation of gates 251' and 25g is made more nearly coincident in time. After some dozens of prints have been made from: rather similar negatives, a setting of 0 is reached at which the gates 25r and 25g operate almost simultaneously. Thereafter the direction of rotation of motor 62 reverses after each successive negative or group of negatives has been printed.

In a similar manner, motor 72 adjusts the proportion 0 of winding 107 of potentiometer 2% which is selected by sliding contact 106. The value of 0,, will be adjusted automatically to cause gate 251? to operate substantially in coincidence with gates 25r and 25g.

In FIG. 14 motor 62 is depicted mechanically coupled not only to moving contact 1101 of potentiometer 29r, but also to moving contact 103 of potentiometer 104 in time ratio controller 94. Contact 103 selects a proportion 0 of the total resistance R of potentiometer 104. The resistance O R fulfils in controller 94 the same purpose as resistance 43 in the circuit shown in FIG. 10. References bearing a sufiix r in unit 94 of FIG. 9 refer to components corresponding to similarly numbered components in the circuit of FIG. 10. Considering the time ratio controller 94, therefore, it will be seen that if gate 251' remains in the 0 condition for time 1,, the output of controller 94 will remain in the 0 condition for a time t where:

r' 100+ r) 104 ioi'l's rioe tr'=tr I' 104 r' 104 But I is given by:

t =A.0,V (II) where V is the reference voltage delivered by unit 281' and A is a constant.

Hence From Equation 'I-II it is evident that the actual exposure time, t,', to red light is independent of 6,, but may be controlled by adjusting p Although the action of motor 62 adjusts the value of 0 to cause t to equal the corresponding time t for which gate 25g is in the 0 condition, motor 62 produces no direct effect on the duration t, of red light exposure.

Time ratio controller 96 in the blue exposure control channel contains components 108-11 1 and 42b, 44b, 46b, 47b, 48b, corresponding to components 103-106 and 42r, Mr, 461', 47r, 48r, respectively of unit 94 in the red exposure control channel. By exactly similarly reasoning, it may be shown that the blue-light exposure time, 11 is given by:

where V;, is the reference voltage delivered by unit 28b and C is a constant.

Motor 72, acts to maintain the time t for which gate 25b is in the 0 condition substantially equal to z or t whichever is the longer.

In a similar way, it will be seen that the green exposure time, t is given by:

From Equations III, IV, and VI, it is seen that the proportions of t',, t and t may be adjusted for any negative by adjustment of 11,, 1,0, and l/l Moreover, although imotors 62 and 72 act to maintain substantial equality between t t and t the values of t' 1f and t' are not affected by such action.

The circuit shown in FIG. 9 may additionally be connected to that of FIG. 8, terminals 31'-35' being connected to terminals 31-35 respectively. The circuit of FIG. 9 will then function as previously described except that the ratios controlled, t zt n are not necessarily identical with the ratios of exposure times, t zt zt' However, if rp p 1%, are not changed, the same purpose is fulfilled as before, viz. the exposure times are inhibited from departing by more than predetermined proportions from previously established ratios.

In addition the following procedures may also be adopted:

(i) A photoelectric integrator for white light is set up to operate more quickly than the red, green or blue integrators. Separate time ratio controllers are used which provide different values of q/ p and thus set difrerent maximum time limits on t,, t' and t the exposure times to red, green and blue light.

(ii) As in (i) but another three time ratio controllers are used to set minimum time limits also.

(iii) In a simple method which assumes a dead-heat condition on an average negative, only one time ratio controller is used, a time interval 2 ends with the end of t z or z (whichever is the first to end). At end of t(1+q/p) the other two exposures are completed.

(iv) In an alternative embodiment of the invention the time ratio controller is a reversible counter, stepped forward at one frequency and back again at another.

Whereas the embodiments described with reference to FIGS. 6-13 have all related to printers of known type using time-wise adjustment of exposure of print material to red, green and blue light components, the invention is equally applicable to another known class of printer in which the intensities of red, green and blue printing light are varied in mutual proportion to establish a predetermined proportion of intensities incident on the print material, the exposure time to all three colours being the same. Some printers have been described in which balance of red, green and blue light intensities is adjusted manually (Hunt, J. Photographic Science, 11, pp. 117-120, 1963). It is clearly a simple matter to provide servo control of the balance 30 that adjustment is automatic. 

